1. Field of the Invention
The present invention relates to a gear speed reducer for reducing the rotational speed of an actuator that is energized by an electric signal and outputting the reduced rotation.
2. Description of the Related Art
There have heretofore been employed in the art gear speed reducers for reducing the rotational speed of an actuator that is energized by an electric signal of a motor or the like, and transmitting the reduced rotation to an output member.
For example, such a gear speed reducer is combined with a throttle valve. The rotational output power from an electric motor is transmitted through a drive gear comprising a spur gear to an intermediate gear. After the rotation of the drive gear is reduced in speed by the mesh between the drive gear and the intermediate gear, the rotation is transmitted through a driven gear held in mesh with the intermediate gear to the valve shaft of the throttle valve. The rotational output power from the electric motor is reduced and transmitted through the drive gear, the intermediate gear, and the driven gear to the valve shaft, thereby rotating the valve shaft. For details, reference should be made to Japanese Laid-Open Patent Publication No. 2003-206762, for example.
There is also known in the art a worm gear speed reducer which employs a worm gear for reducing and transmitting rotational output power from an actuator such as an electric motor therethrough to an output shaft. The worm gear speed reducer has a worm mounted on the motor shaft of the electric motor and a helical gear supported on a shaft that extends substantially perpendicularly to the motor shaft and held in mesh with the worm. The drive torque from the electric motor is transmitted from the motor shaft to the worm and then from the worm to the helical gear, and the reduced drive toque is output to the output shaft. For details, reference should be made to Japanese Laid-Open Patent Publication No. 2003-172407, for example.
The speed reducer disclosed in Japanese Laid-Open Patent Publication No. 2003-206762 employs spur gears as the drive gear for transmitting the output of the electric motor, the intermediate gear held in mesh with the drive gear, and the driven gear held in mesh with the intermediate gear for transmitting the rotation to the valve shaft. If the rotational output power from the electric motor is to be output to the output shaft with largely reduced speed, then it is necessary to increase the number of gear teeth of the gears. However, an increase in the number of gear teeth of a spur gear results in an increase in the diameter of the spur gear. Furthermore, since the drive gear, the intermediate gear, and the driven gear are supported respectively on support shafts extending substantially parallel to each other, the speed reducer including these gears tends to become larger in size.
The worm speed reducer disclosed in Japanese Laid-Open Patent Publication No. 2003-172407 employs the worm gear which comprises the worm and the helical gear as a speed reducer mechanism. As the worm gear has poor rotation transmitting efficiency, it is difficult to transmit the rotational output power from the electric motor efficiently to the output shaft. In addition, the worm gear is low in durability.
The motor shaft on which the worm is mounted and the output shaft of the helical gear that is held in mesh with the worm extend substantially perpendicularly to each other. Therefore, the worm speed reducer is large in size because of the space required therein for housing the motor shaft and the output shaft, and a large installation space is needed for installing the worm speed reducer.